This invention relates to apparatus for switching electric current, such as direct current (DC) electricity; and more particularly to such apparatus which has a mechanism for extinguishing arcs formed between switch contacts during separation.
DC electricity is used in a variety of applications such as battery powered systems, drives for motors and accessory circuits, in which contactors are used to make and break the flow of current to the load. Weight, reliability and high DC voltage switching and interrupting capability are important considerations in developing the contactor.
In many applications relatively large direct currents must be switched which produce arcs when the contacts of the contactor separate, thereby requiring a mechanism for extinguishing the arcs. Previous DC contactors and switches incorporated one or more arc extinguishing chambers, often referred to as "arc chutes" such as described in U.S. Pat. No. 5,866,864, to extinguish arcs that formed between the switch contacts. Arc extinguishing chambers may comprise a series of spaced apart electrically conductive splitter plates.
The self magnetic field produced by current flowing through conductors in the contactor interacts with the arc creating a Lorentz force that drives the arc towards the extinguishing chamber. In DC switching devices, permanent magnets on the sides of the series of splitter plates establish another magnetic field across the entire arc extinguishing chamber which assists the self-field to drive the arc off the contacts and direct the arc into the splitter plate arrangement. The arc then propagates from one splitter plate to another in the series and eventually spanning a number of gaps between the splitter plates whereby sufficient arc voltage is built up that the arc is extinguished.
The disadvantage of using permanent magnets is that the contactor is polarized in that arc current flowing in only one direction produces a Lorentz force in a direction that drives the arc into the extinguishing chamber. The Lorentz force produced by arc current in the opposite direction inhibits the arc from moving toward and into the second extinguishing chamber. A common contactor has a pair of stationary contacts and a movable bridging contact with separate arc extinguishing chambers for each stationary contact. In this contactor, the direction of the DC current determines which arc chamber is active in a bidirectional contactor with permanent magnets. However, it is desirable to provide arc extinction which is not dependent upon the polarity of a permanent magnet. This allows both arc chambers to be simultaneously active thus allowing the interruption of twice the magnitude of source voltage, in a non-polarized (bidirectional) operating mode, than that achievable in prior permanent magnet based bidirectional contactors.